1. Field of the Invention
The present invention relates to a method for determining current in a polyphase machine.
2. Description of the Related Art
Electrical rotating field drives are generally known. As described in the German Patent Application DE 10 2010 001 774, different types of modulation are used. Pulse width modulation (PWM) is commonly used in rotating field drives. In such rotating field drives, a feedback control is carried out as a function of the particular requirements in accordance with one of the following methods:                use of a sinusoidal current control (sinusoidal commutation);        use of a square-wave current control (square-wave current supply);        use of square-wave voltages (control by square-wave voltages);        use of a sinusoidal voltage control with superimposed zero voltages.        
In principle, the aforementioned techniques can be used for electrical machines having any given number of phases. Most frequently encountered in practice are electrical machines having three phases. However, there are also electrical machines that have a different number of phases, such as one, two, four, five, six, seven or nine phases.
Moreover, what are generally referred to as start-stop systems are generally known. These serve the purpose of stopping and restarting a combustion engine in order to reduce fuel consumption and exhaust emissions.
A start-stop system developed by the Applicant operates on the basis of conventional starters. The starter in question is driven by an electronic control unit and engages via a pinion into a ring gear provided on the flywheel.
A starter generator operated in belt drive (RSG) has also already been realized that is based on a claw-pole generator having an additional electronic control unit. In the case of such starter generators, the phases are often directly connected via electronic semiconductor switches to a DC-voltage source, for example, a battery, without the use of a clock pulse method, such as PWM, for example.
To be able to recuperate higher energies during braking, there is a need for higher-voltage systems. To start the combustion engine at higher voltages, a pulsing of the supply voltage in the converter is needed in order to limit the current in the machine to a predefined maximal value. A clocked converter requires a DC link having high-capacitance capacitors in order to smooth the alternating components in the input current. In the output stages, the space requirements for the particular output stage are often determined by the dimensioning of the DC links.
As had been explained above in connection with the mentioned machines, operating modes that employ clock pulse methods (PWM) at higher voltages and operating modes that employ block commutations (without PWM) at lower voltages or at higher speeds are used. In the clock pulse methods, a current measurement is of particular importance since, in these methods, a current control is carried out that requires measuring the actual currents.
To measure current in the individual phases of the electrical machine, either shunt resistors are used that convert the current into an equivalent voltage signal, or indirect current measurement methods are used, such as those that employ current transducers or LEM transducers, for example.
FIG. 1 shows a DC voltage source, for example, a five-phase electrical machine connected to a battery B, that has a generator 1, a power electronics 2, and a DC link 3. In the interior thereof, generator 1 has five phase connections that are linked via phase conductors to branches Z1 through Z5 of power electronics 2. As indicated in FIG. 1, phase currents I_X through I_Y flow in the phase conductors.
Branch Z1 of power electronics 2 contains a high-side switch HS1 and a low-side switch LS1 and is connected at a connection point between these two switches via a phase conductor to generator 1.
Branch Z2 of power electronics 2 contains a high-side switch HS2 and a low-side switch LS2 and is connected at a connection point between these two switches via a phase conductor to generator 1.
Branch Z3 of power electronics 2 contains a high-side switch HS3 and a low-side switch LS3 and is connected at a connection point between these two switches via a phase conductor to generator 1.
Branch Z4 of power electronics 2 contains a high-side switch HS4 and a low-side switch LS4 and is connected at a connection point between these two switches via a further phase conductor to generator 1.
Branch Z5 of power electronics 2 contains a high-side switch HS5 and a low-side switch LS5 and is connected at a connection point between these two switches via a further phase conductor to generator 1.
DC link 3 of the illustrated machine has a DC-link capacitor 3a. 
The switches of power electronics 2 receive the control signals from a control unit (not shown) in order to generate a rotating field upon machine startup.
As is readily apparent from FIG. 1, the following relation holds for the currents flowing in the machine:I_Bat=1_Gen+I—ZK 
In this context, I_Bat is the current of the DC voltage source; I_Gen is the generator current; and I_ZK is the DC link voltage.
In the case of an electrical machine as shown in FIG. 1, it is already known for current measurements to be taken using shunt resistors in the phase conductors between branches Z1 through Z5 and generator 1. This is illustrated in FIG. 2. Here, the disadvantage is that very high power losses occur in all operating modes of the electrical machine. They lead to unwanted further heating of the output stage region during the entire working operation of the machine. This is because the full operating current of the machine flows through the shunt resistors.
Moreover, in the case of an electrical machine as shown in FIG. 1, it is already known for current measurements to be taken using shunt resistors that are configured in the branches of power electronics 2, for example, in the low-side region of the particular branch. This is illustrated in FIG. 3. This is also associated with the disadvantage that very high power losses occur in all operating modes of the electrical machine that lead to unwanted additional heating of the output stage region during the entire working operation of the machine. This is because the full operating current of the machine flows through the shunt resistors.
The current measurements illustrated in FIGS. 2 and 3 are associated with the further disadvantages that a comparatively high number of evaluation circuits must be made available, and it is not possible to measure the current of the DC voltage source using this configuration. It is merely possible in these configurations to estimate the current of the DC voltage source as a function of the measured phase currents and the activation durations.